1. Field of the Invention
The present invention generally relates to a self-calibration method for use in a mobile transceiver, and more particularly to a method for self-calibrating a Direct Current (DC) offset and a mismatch between orthogonal signals occurring in a mobile transceiver.
2. Description of the Related Art
A mobile transceiver may be fundamentally degraded by non-ideal characteristics such as a Direct Current (DC) offset and mismatch (or in-phase/quadrature phase (I/Q) imbalance).
The DC offset is caused by self-mixing of a mixer provided in the mobile transceiver. The DC offset occurs when a local oscillator (LO) signal leaks inducing an input to an antenna and is subsequently fed back from the antenna or when a radio frequency (RF) modulation signal input to the antenna leaks to a LO. In this case, the DC offset value saturates a baseband (BB) circuit.
A fault occurring in the circuitry of the oscillator with a phase delay device and a line for connecting the oscillator and the mixer causes the mismatch. This is because the phase difference between I and Q channel signals generated from the oscillator of the mobile transceiver does not become 90 degrees. The mismatch can be reduced if mixers of I and Q channel demodulators are designed to be symmetrical to each other. However, there is a problem in that current consumption as well as a mixer size increases when the mixers are designed to be symmetrical. This mismatch decreases the signal-to-noise ratio (SNR) and therefore increases a bit error rate (BER). As a result, performance of the mobile transceiver is degraded.
Thus, a need exists for a method for estimating and calibrating the DC offset and the mismatch to improve the performance of the mobile transceiver.
FIG. 1 is a circuit diagram illustrating an example of independently estimating and calibrating a mismatch and DC offset in a conventional mobile transceiver. The example of FIG. 1 is described in PCT International Publication Number WO 2004/023667 entitled “Direct-Conversion Transceiver Enabling Digital Calibration” and an article entitled “New Methods for Adaptation of Quadrature Modulators and Demodulators in Amplifier Linearization Circuits” by James K. Cavers.
For convenience of explanation, the estimation path is not divided into I and Q channel paths in FIG. 1. However, the same proposition holds true even when the estimation path is divided into the I and Q channel paths.
According to the method proposed in FIG. 1, all mismatches and DC offsets occurring in transmission (TX) and reception (RX) stages are calibrated. For this, the calibration for the TX stage is first performed and then the calibration for the RX stage is performed. That is, the calibration for the TX stage should be first performed before the calibration for the RX stage. The calibration for the TX stage is the TX IQ calibration. The calibration for the RX stage includes calibration of the DC offset as well as calibration of the mismatch between I and Q channels.
The estimation method using FIG. 1 uses a discrete envelope detector. The envelope detector converts an envelope signal output from a driver amplifier of the TX stage into a baseband (BB) signal, and takes the discrete Fourier series for the complex envelope waveform of the BB signal. The envelope detector estimates gain imbalance, phase imbalance and the DC offset of each of the I/Q channels in the TX stage using the discrete Fourier series.
In the case of the above-described estimation method, non-ideal factors of the envelope detector should be known. The non-ideal factors are differential gain and a DC value. In the above-described article and PCT International Publication Number WO 2004/023667, the non-ideal factors are estimated.
In the above-described method, the gain imbalance, the phase imbalance and the DC offset of each of the I/Q channels may not be correctly estimated in the TX and RX stages. As seen from FIG. 1, an increased number of diodes, resistors, capacitors and switches are additionally required to configure the envelope detector.